I've ran an optimize on this table (which is a recreate for Innodb engine), which caused the original user_id foreign key constraint to disappear. I've re-added the constraint and I am no longer hitting the traceback. I am still investigating why this may have happened to begin with but not seeing much of anything in mysql logs.

It looks like this was added by migration 792. It dropped and re-added `collections_subscriptions_ibfk_2`, but the other constraint was called `collection_subscriptions_ibfk_2`, so it stuck around.
Thanks for looking into this!